The present invention is related to the removal of surface radioactive contamination from metallic materials and the collection of the removed contamination in a form suitable for convenient radioactive waste treatment and disposal. More specifically the invention relates to the dissolution of surface contamination with an acid solution, the subsequent conversion of the dissolved ions into metal particles by means of an electrochemical cell and the removal of the metal particles from the decontamination system.
Chemical decontamination has become an established method for reducing the radiation exposure of workers at nuclear plants. In this method chemicals are added to a part of the cooling water circuit of the plant and these chemicals dissolve radioactive deposits from the circuit surfaces. The chemicals and the radioactive components are removed from the circulating cooling solution by ion exchange. An example of such a processes is disclosed in U.S. Pat. No. 4,705,573 titled xe2x80x9cDescaling Process.xe2x80x9d
More recently, the same general principle has been applied to the treatment of components which are no longer required for service in nuclear plants. The decontamination of these components not only reduces the worker radiation exposure, but also reduces the radioactive level so that the decontaminated components may be treated as a lower category of radioactive waste. If the decontamination process reduces the radioactivity below a certain level, the components can be regarded as non-radioactive. These processed components may then be reused as components in other nuclear or non-nuclear applications or recycled. This decontamination processing is sometimes referred to as Decontamination for Decommissioning. An example of this kind of process is disclosed in U.S. Pat. No. 6,147,274 titled xe2x80x9cMethod For Decontamination Of Nuclear Plant Components.xe2x80x9d
The existing Decontamination for Decommissioning processes expose contaminated metal components to a decontamination solution which removes a layer of material. The radioactive solids and cations are then removed from the decontamination solution. The decontamination for decommissioning processes do not generate liquid radioactive waste because the water used to make up the decontamination solutions is returned to a pure deionized form at the end of the process and can be recycled. The decontamination processes also use dilute solutions and avoid the hazards associated with using concentrated chemicals. The decontamination processes are particularly useful for cleaning components of complex shape (such as tube-in-shell heat exchangers) where mechanical decontamination methods are difficult to apply.
A significant problem with existing Decontamination for Decommissioning processes has been the difficulty of handling the secondary radioactive waste. In some countries radioactive waste burial facilities do not exist and it is necessary to store any secondary waste generated indefinitely on-site. For secondary radioactive waste removal the waste must be transferred to a storage facility where storage container integrity and radiation shielding can be guaranteed. The secondary radioactive waste must also be in a form suitable for transportation and burial. In one such method, the secondary radioactive waste is conditioned as a solid monolith and contained in easily handled drums. In all countries it is imperative to maximize the stability and minimize the volume of secondary radioactive waste produced.
Decontamination for decommissioning processes usually generate ion exchange resins as the final secondary radioactive waste form. All the radioactivity from the decontaminated components and any residual chemicals are collected in this ion exchange resin waste. In the United States radioactive ion exchange resin waste is routinely de-watered and sent for burial in high integrity containers. In other countries regulations prohibit handing radioactive ion exchange resin waste in this manner. The resin waste is also not a convenient waste form because it only holds a fraction of its own weight in the form of radioactive or metallic contamination. Thus, the final radioactive resin waste does not consist of just the contamination removed from the component surfaces, but also the organic polymeric materials which make up the ion exchange resin itself. This inefficiency in the waste processing of resin based decontamination is a significant drawback when compared to mechanical decontamination methods where a thin layer of contaminated material is removed from the component surfaces. The only waste produced by mechanical decontamination methods is the material removed from the component.
U.S. Pat. No. 5,078,842A titled xe2x80x9cProcess For Removing Radioactive Burden From Spent Nuclear Reactor Decontamination Solutions Using Electrochemical Ion Exchangexe2x80x9d discloses a process in which ion exchange resin can be used as an intermediate waste form and is hereby incorporated by reference. The ""842 patent discloses a three compartment electrochemical ion exchange cell used to remove radioactive cations from a decontamination solution. A decontamination solution passes through a central compartment of the ion exchange cell containing cation exchange resin. The cation exchange resin removes dissolved contaminants and metal ions from the decontamination solution. The ions held on the resin then migrate under the influence of an electric current into a cathode compartment and reduced to a metallic deposit on a cathode. An anionic equivalent of this process is disclosed in U.S. Pat. No. 5,306,399 titled xe2x80x9cElectrochemical Exchange Anions In Decontamination Solutionsxe2x80x9d and is also hereby incorporated by reference.
Although the method described in the ""842 patent was extensively tested, the process has not been applied on a full commercial scale for nuclear reactors. A reason for the lack of use is that the process was designed to be part of operational reactor decontamination, which was the subject of commercial interest in the past. Operational decontamination takes place during the nuclear plant maintenance shut down and must be performed in a very short time. Indeed, maintenance shut-downs have become so short at nuclear plants that there is often insufficient time for the decontamination process using a ion exchange cell to be accommodated, let alone an ancillary decontamination solution treatment system. These time constraints require the electrical migration process to happen very quickly, which in turn required a large input of electric power to the ion exchange cell together with heavy, expensive equipment which was commercially unattractive.
The decontamination process disclosed in the ""842 patent is much more suited to decontamination for decommissioning applications, where waste volume reduction is imperative and the time constraints are less onerous. In this case the electrical process can take place at low current with modest equipment over a longer period of time. Of crucial importance, the ""842 patent does not address the manner in which the radioactive deposit formed on the cathode should be handled. On a laboratory scale the cell described could then be dismantled and the deposit removed mechanically from the electrode surface. Such a method is not feasible at full scale within proper radiation protection constraints. For the method to be fully commercialized it would be necessary to devise a new method for removing the radioactive deposit on the cathode from the ion exchange cell for disposal.
U.S. Pat. No. 4,828,759 titled xe2x80x9cProcess For Decontaminating Radioactivity Contaminated Metallic Materialsxe2x80x9d discloses yet another decontamination process in which an acidic decontamination solution is used to remove radio active materials from components. The contaminants and solid impurities are subsequently removed from the decontamination solution by processing with an ion exchange cell through electrochemical means and deposited on a cathode. Like the ""842 patent, the ""759 patent does not address the manner in which the radioactive deposits are removed from the cathode or how the radioactive waste is processed for disposal.
The invention is a system and method for the chemical decontamination of radioactive metallic objects. The inventive decontamination system only produces solid radioactive waste materials in the form of small metal particles and all liquids used in the inventive decontamination system may be recycled. The contaminated metallic objects are exposed to an acid decontamination solution which removes a thin layer of contaminated material. An electrochemical ion exchange is then used to remove radioactive components from the decontamination solution.
The radioactive contamination is then removed from the decontamination solution. After the decontamination solution has absorbed the radioactive materials, the decontamination solution flows through a purification loop. The decontamination loop has an ion exchange cell having a central compartment, an anode compartment and a cathode compartment. This loop may also have a filter which removes solid substances from the decontamination solution released from the components during the decontamination process.
In an embodiment, the ion exchange cell is configured such that the anode and cathode compartments are each separated from the central compartment by ion permeable membranes. The central compartment is filled with cation exchange resin. The decontamination solution flows through the central compartment and the radioactive metal cations in the decontamination solution are captured on by the resin. The trapped radioactive metal cations then migrate under the influence of electric current into the cathode compartment where they are deposited in the form of small metal particles on a cathode. The purified decontamination solution flows out of the central compartment and may be recycled for additional component decontamination.
In an alternative embodiment, the ion exchange cell described above is not filled with a cation exchange resin and the cations in the decontamination solution flowing through the central chamber are not trapped by resin. In this embodiment, radioactive metal cations migrate under the influence of electric current from the decontamination solution through an ion permeable membrane into the cathode compartment. The metal cations then are deposited in the form of small metal particles on a cathode. Not all of the radioactive cations may be removed from the decontamination solution, however a sufficient amount of contamination is removed so that the decontamination solution flowing out of the ion exchange cell can be reused to decontaminate other components.
In both embodiments, a cathode solution flows over the cathode and removes the contaminated metal particles from the cathode. The cathode solution and metal particles flow out of the cathode compartment and into a waste collection container where the metal particles settle out of the solution. After the cathode solutions is free of radioactive contaminants it may be recycled.
In yet another embodiment, the central compartment of the electrochemical ion exchange cell is not filled with cation exchange resin and the cathode compartment is not separated from the central compartment by an ion permeable membrane. The decontamination solution flows from the central compartment into the cathode compartment. In this embodiment the decontamination solution does not flow out of the central compartment. The radioactive metal cations migrate directly to the cathode and are deposited on the cathode as small metal particles. The cathode solution flows over the cathode and removes the deposited metal particles. Both the cathode solution and the decontamination solution flow into the waste collection container. Again the metal particles settle out of the mixed solution. The solution in the waste collection container is recycled as both cathode solution and decontamination solution.
The radioactive metal deposited on the cathode is in the form of small particles rather than an adherent layer by controlling the acidity of the solution in the cathode compartment. In an embodiment, the pH level in the cathode compartment is maintained at about 2.5 to 5.0 which results in small particles being formed on the cathode. These small particles are easily detached from the cathode by a liquid flowing over the cathode and are also large enough to be easily separated from the cathode solution. As discussed, the metal particles settle to the bottom of the waste collection container allowing the solution to be removed from the upper part of the container and recycled.
An object of the present invention is to minimize the quantity of radioactive waste generated by the decontamination process. The metallic contaminants from the components are converted into small metallic particles which are gathered in a waste collection container. By only producing solid metal particle waste the minimum possible waste volume is generated.
The metal particles can then be moved from the waste collection container to another location by hydraulic fluidization, similar to the way radioactive ion exchange resin is conventionally handled. This feature of the invention provides a practical method for removing the radioactive contamination from the apparatus. If a coherent metal deposit were formed on the cathode the only practical way to remove the contamination would be to physically remove the cathode from the apparatus, which would be difficult to achieve in a radiologically safe manner.
The inventive process for removing radioactive metal waste from an electrode is also compatible with other ion exchange cell decontamination processes such as that described in U.S. Pat. No. 6,147,274 as well as other types of dilute acid decontamination processes.